1. Field of the Invention
This invention generally relates to wireless communications antennas and, more particularly, to a selectable antenna array formed from a planar field of microelectromechanical switches (MEMSs).
2. Description of the Related Art
The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems while reducing their size, or placing these components in less desirable locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver.
Wireless telephones can operate in a number of different frequency bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band, at around 1900 MHz, are used. Other frequency bands include the PCN (Personal Communication Network) at approximately 1800 MHz, the GSM system (Groupe Speciale Mobile) at approximately 900 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are GPS signals at approximately 1575 MHz and Bluetooth at approximately 2400 MHz.
Conventionally, good communication results have been achieved using a whip antenna. Using a wireless telephone as an example, it is typical to use a combination of a helical and a whip antenna. In the standby mode with the whip antenna withdrawn, the wireless device uses the stubby, lower gain helical coil to maintain control channel communications. When a traffic channel is initiated (the phone rings), the user has the option of extending the higher gain whip antenna. Some devices combine the helical and whip antennas. Other devices disconnect the helical antenna when the whip antenna is extended. However, the whip antenna increases the overall form factor of the wireless telephone.
It is known to use a portion of a circuitboard, such as a dc power bus, as an electromagnetic radiator. This solution eliminates the problem of an antenna extending from the chassis body. Printed circuitboard, or microstrip antennas can be formed exclusively for the purpose of electromagnetic communications. These antennas can provide relatively high performance in a small form factor. However, a wireless device that is expected to operate at a plurality of different frequencies may have difficulty housing a corresponding plurality of microstrip antennas. Even if all the microstrip antennas could be housed, the close proximity of the several microstrip antennas may degrade the performance of each antenna.
In some circumstances it is advantageous to be able to shape an antenna pattern. Then, the antenna pattern has additional gain in a desired direction, to improve the link margin with a communicating device. It is known to network a plurality of antenna elements and regulate the phase relationship between elements. The phase relationship between elements generates the antenna beam pattern. Likewise, an active element can be arrayed in a field, or lattice of parasitic elements. These parasitic elements, being either half-wavelength open radiators or quarter-wavelength ground-shunted radiators, can also be used to shape an antenna beam pattern. Unlike the phase-array antenna, whose pattern can easily be varied by electronic means, the parasitic elements must be manipulated by mechanical means if the beam is to shaped in a different form. Mechanical manipulation generally requires additional parts that take up room and degrade reliability. As a result, parasitic element lattices have not been practical for use in portable wireless communication devices.
FIG. 28 is a schematic diagram of a microelectromechanical switch (MEMS) (prior art). A MEMS is a semiconductor integrated circuit (IC) with an overlying mechanical layer that operates as a selectable connectable switch. That is, the underlying solid-state layer creates a field that can cause an overlying conductive material to move, permitting the conductive material to act as miniature single-pull single-throw switch. MEMS concepts were developed in labs in the 1980""s and are just now beginning to be fabricated as practical products. As a result, the particular specifications and features of a MEMS are still under development. MEMS technology offers the possibility of extremely low loss switches miniature switches.
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. PIN diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value of under 20 dB. This allows a signal to xe2x80x9cbleedxe2x80x9d across the switch even when the switch is open. PIN diodes and transistors have a limited frequency response and typically only respond to frequencies under 20 GHz. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
As noted in U.S. Pat. No. 6,440,767 (Loo et al.), a microwave MEMS can be made utilizing an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature. This switch provides only a single-pole, single-throw (SPST) function, that is, the switch is either open or closed.
A SPST MEMS switch can be formed from a multiple-layer armature with a suspended biasing electrode and a conducting transmission line affixed to the structural layer of the armature. A conducting dimple is connected to the conducting line to provide a reliable region of contact for the switch. The switch is fabricated using silicon nitride as the armature structural layer and silicon dioxide as a sacrificial layer supporting the armature during fabrication.
A MEMS switch suitable for RF or microwave applications typically can have a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB) over a large bandwidth, as compared to semiconductor transistors and pin diodes. These characteristics give the MEMS switch the potential to not only replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits, but to create a whole new class of high performance and compact microwave switch circuits. RF signals often must be switched between two destinations, such as when switching an RF signal between a first antenna array and a second antenna array. Switches that support this configuration are classified as single-pole, double-throw (SPDT) switches.
It would be advantageous if a single wireless communications telephone antenna could be made to operate at a plurality of frequencies using MEMS devices.
It would also be advantageous if the antenna beam pattern of the above-mentioned multi-frequency MEMS antenna could be controlled.
It would be advantageous if the MEMS devices could be used to vary the electrical length of parasitic elements in a parasitic element antenna array.
It would be advantageous if the phase relationship between a MEMS antenna active element and the parasitic elements could be controlled to beam shape the antenna pattern.
It would be advantageous if the MEMS antenna could be fabricated on a single planar surface or a dielectric sheet.
The present invention provides a microstrip, or printed circuitboard antenna that is made with MEMSs to vary the actual physical length of the printed line active element radiators. The MEMSs can be used to form selectable connected conductive sections that vary the length, position, and shape of the antenna active element. In this manner, the antenna operating frequency, bandpass, or beam pattern can be modified. In addition, the active element is situated in a lattice of MEMS parasitic elements. The MEMS devices in the parasitic elements serve multiple purposes. The length of the parasitic element can be modified to operate at different frequencies. The position and distance with respect to the active element can be modified to change antenna beam pattern. Since both the active and parasitic elements are formed on a common, planar surface, the antenna is relatively easy to manufacture. Further, the thin profile presented by the planar structure is relatively small, permitting the antenna to be used in portable electronic devices.
Accordingly, a MEMS planar antenna array is provided comprising a planar field of MEMSs. In one aspect of the antenna, a lattice of parasitic elements is formed by selectively connecting at least one MEMS in the field. In another aspect, an antenna active element is formed by selectively connecting a MEMS in the field. Alternately, both the parasitic elements and the active elements are formed by connecting MEMSs.
The parasitic elements have a number, shape, length, and distance from the active element, and a position with respect to the active element that are formed in response to selectively connecting MEMS in the field. Further, a plurality of parasitic element lattices are formed in response to selectively connecting MEMS in the field. Likewise, the active element has a length, shape, and position that is formed in response to selectively connecting MEMS in the field. Patch, monopole, and dipole antennas are among the antenna types that can be formed from the MEMS.
Additional details of the above-described planar antenna array and a method for forming a antenna array from a planar field of MEMS are provided below.